In reply to GermanAlex:
The nose is remarkably sensitive outdoors in the fresh air when it's not being bombarded with hundreds of other molecules and trying to ingore most of them, unless they smell dangerous or edible.
We live a few miles from Clitheroe cement works and when the wind is from that direction there's a strong smell, hard to describe but mostly sulphur dioxide. During the first big storm a week ago I went outside and noticed a strong odour of SO2 in the air, which is strange because between us and the Atlantic is Preston and Ireland and I'm not aware of any heavy industries in Preston. Somebody else, living in the Bristol area, mentioned it on Cycle Chat the following day so I wondered if we were picking up pollution from the American east coast. The wind must be flowing in almost a laminar way across the Atlantic at the moment, undisturbed by vortices. I emailed the Met Office but don't expect a reply.
One other possibility is that in Preston around the east side of the town there is often an unpleasant and distinctive chloriney smell, almost with elements of chocolate in it. We have tried to work out its source but one thing's for sure, which is that I used to smell exactly the same odour in Gosforth, Newcastle upon Tyne, when I lived there and I always understood that it came from a big pharmaceutical factory somewhere to the west. Maybe there's something like that near where you were walking?